What's wrong with my Parents XP Pro Desktop?

The pc might have a virus, malware. Try to determine and make sure that it doesn't have any viruses. Now once that is determined and the pc is still slow, start uninstalling unneccessary programs, files taking up space, and optimize the pc. That includes defragmenting, and even if it takes forever, let it finish and everybody in the end will be happy. Hope you fix it.